Update to release_3 in installation.md (#4144)

Author: andrewcoh

com.unity.ml-agents/Documentation~/com.unity.ml-agents.md

@@ -114,7 +114,7 @@ a number of ways to [connect with us] including our [ML-Agents Forum].
 [unity ML-Agents Toolkit]: https://github.com/Unity-Technologies/ml-agents
 [unity inference engine]: https://docs.unity3d.com/Packages/com.unity.barracuda@latest/index.html
 [package manager documentation]: https://docs.unity3d.com/Manual/upm-ui-install.html
-[installation instructions]: https://github.com/Unity-Technologies/ml-agents/blob/release_2_docs/docs/Installation.md
+[installation instructions]: https://github.com/Unity-Technologies/ml-agents/blob/release_3_docs/docs/Installation.md
 [github repository]: https://github.com/Unity-Technologies/ml-agents
 [python package]: https://github.com/Unity-Technologies/ml-agents
 [execution order of event functions]: https://docs.unity3d.com/Manual/ExecutionOrder.html

docs/Installation-Anaconda-Windows.md

@@ -123,10 +123,10 @@ commands in an Anaconda Prompt _(if you open a new prompt, be sure to activate
 the ml-agents Conda environment by typing `activate ml-agents`)_:
 
 ```sh
-git clone --branch release_2 https://github.com/Unity-Technologies/ml-agents.git
+git clone --branch release_3 https://github.com/Unity-Technologies/ml-agents.git
 ```
 
-The `--branch release_2` option will switch to the tag of the latest stable
+The `--branch release_3` option will switch to the tag of the latest stable
 release. Omitting that will get the `master` branch which is potentially
 unstable.
 

docs/Installation.md

@@ -57,18 +57,18 @@ example environments and training configurations to experiment with them (some
 of our tutorials / guides assume you have access to our example environments).
 
 ```sh
-git clone --branch release_2 https://github.com/Unity-Technologies/ml-agents.git
+git clone --branch release_3 https://github.com/Unity-Technologies/ml-agents.git
 ```
 
-The `--branch release_2` option will switch to the tag of the latest stable
+The `--branch release_3` option will switch to the tag of the latest stable
 release. Omitting that will get the `master` branch which is potentially
 unstable.
 
 #### Advanced: Local Installation for Development
 
 You will need to clone the repository if you plan to modify or extend the
 ML-Agents Toolkit for your purposes. If you plan to contribute those changes
-back, make sure to clone the `master` branch (by omitting `--branch release_2`
+back, make sure to clone the `master` branch (by omitting `--branch release_3`
 from the command above). See our
 [Contributions Guidelines](../com.unity.ml-agents/CONTRIBUTING.md) for more
 information on contributing to the ML-Agents Toolkit.
@@ -87,7 +87,7 @@ please follow the [advanced installation instructions](#advanced-local-installat
 #### Advanced: Local Installation for Development
 
 You can [add the local](https://docs.unity3d.com/Manual/upm-ui-local.html)
-`com.unity.ml-agents` package (from the repository that you just cloned) to our
+`com.unity.ml-agents` package (from the repository that you just cloned) to your
 project by:
 
 1. navigating to the menu `Window` -> `Package Manager`.

docs/Training-on-Amazon-Web-Service.md

@@ -69,7 +69,7 @@ After launching your EC2 instance using the ami and ssh into it:
 2. Clone the ML-Agents repo and install the required Python packages
 
    ```sh
-   git clone --branch release_2 https://github.com/Unity-Technologies/ml-agents.git
+   git clone --branch release_3 https://github.com/Unity-Technologies/ml-agents.git
    cd ml-agents/ml-agents/
    pip3 install -e .
    ```